The present invention relates to a transmission system using optical fibers and, more particularly, to a long-distance, large-capacity optical transmission system employing return-to-zero lightwave pulses, such as soliton lightwave pulses, and optical amplifiers.
With the development of optical amplifying techniques, optical fiber communication technology has made rapid-paced progress toward ultra-long-distance communication, now allowing implementation of a long-range communication system without the need of using regenerative repeaters. At increased transmission rates, however, conventional transmission systems suffer serious degradation of their transmission characteristics caused by the wavelength dispersion and nonlinear effect of optical fibers, imposing severe limitations on the realization of a high-speed, high-capacity transmission system.
Usually, when lightwave pulses are transmitted over an optical fiber, their pulse width broadens by the wavelength dispersion characteristic of the optical fiber owing to the frequency spread inherent in data-modulated lightwave pulses. The freedom of a soliton lightwave signal from variation in its pulse waveform by transmission is achieved when the pulse width compression, caused by frequency chirping of the lightwave pulses owing to optical nonlinearities of the optical fiber forming the transmission line, balances with the aforementioned pulse width broadening. To accomplish an optical soliton transmission which maintains the above-mentioned balance and is substantially free from variations in the lightwave pulse waveform, it is necessary that the zero dispersion wavelength of the transmission line be shorter than the wavelength of the lightwave signal to hold a desired (anomalous) wavelength dispersion characteristic.
In an optical soliton transmission which is generally free from waveform degradation by transmission, timing jitter brought about by various causes during transmission is a main factor determining the transmission characteristic, along with degradation of the signal to noise ratio by accumulated optical noises. The Gordon-Haus jitter forms a main part of such timing jitter. In an optically amplified transmission system, the optical soliton carrier frequency which randomly fluctuates due to optical noises produced by optical repeater-amplifiers, is converted mainly by the wavelength dispersion characteristic of the fiber optic transmission line into fluctuations in the system. The Gordon-Haus jitter increases with distance and, hence, exerts a great influence on long-distance soliton transmission. The pulse spacing reduced by the Gordon-Haus jitter also increases the interaction between adjacent optical soliton pulses, newly causing timing jitter. Thus, the Gordon-Haus jitter is an important problem which must be solved in order to transmit long distances.
Since the amount of Gordon-Haus jitter is proportional to the group-velocity dispersion present in the transmission line, decreasing the fiber""s average dispersion reduces the timing jitter. This leads to additional complications, however, as distortions due to four-wave mixing become more pronounced at low dispersion values.
Dispersion management has proven to be an effective technique to reduce both effects simultaneously. The idea of dispersion management is to concatenate fibers of both normal and anomalous dispersion to form a transmission line having both a high local group-velocity dispersion (GVD) and a low path-averaged GVD. This is beneficial since high local dispersion significantly reduces the efficiency of four-wave mixing, decreasing both the modulational instability gain and bandwidth. In addition, lowering the average dispersion also reduces the Gordon-Haus timing jitter of soliton transmission systems. Furthermore, dispersion management has been found to enhance the soliton energy; this additionally reduces the timing jitter below the amount that would be obtained in a system with constant dispersion equal to the path-averaged value.
In high-rate terrestrial soliton transmission systems, intersymbol interference produced by the generation of dispersive waves (or, continuum radiation) shed by propagating pulses is a major limitation. It is therefore important to diminish the shedding of energy from the input pulse into a dispersive pedestal, which can be achieved by launching properly shaped and chirped pulses with optimum power into the fiber. In practice, typical optical sources generate unchirped pulses and input pulse chirping is realized by using an additional piece of fiber preceding the beginning of the transmission line. It is therefore convenient to identify points in each dispersion-map period where pulses are naturally unchirped, and use one such location as the launch point. The partial map period preceding the first complete map period then plays the role of the prechirping fiber.
Therefore, an object of the subject invention is an efficient, fiber-optic communication system.
A further object of the subject invention is a manner of optimizing a dispersion map for use in decreasing dispersive radiation in a fiber-optic system.
These and other objects are attained by the subject invention, which includes dispersion maps with zero-chirp point positions independent of the dispersion values of the types of fiber comprising the map. A zero-chirp point can be used as a special launching point for optical pulses; the subsequent evolution of the pulses is much cleaner than if other launching points are used since there is less dispersive radiation shed by the pulse as it evolves. Because these optimized dispersion maps identify zero-chirp launching points which are independent of the dispersion values of the types of fiber comprising the map, the launching points are, therefore, also independent of the wavelength since the second-order dispersion varies with wavelength. As a result, such a dispersion map allows minimal shedding of dispersive radiation at several frequencies simultaneously, a situation that is ideal for wavelength-division-multiplexing. With this optimized map, one need only choose the appropriate length of the fiber segments, locate the zero chirp point, and cut the combined fiber segments at the zero-chirp point location to form an optimal prechirping fiber. Optimum lengths are chosen by using a theoretical estimate for the imbalance between the effects of the group velocity dispersion and nonlinear index of refraction.